Dental radiology equipment as described in French patent FR 2,547,495 and European patent No. 0,129,451 is known.
Such equipment comprises an x-ray source that emits radiation directed to a tooth located in a patient's mouth and behind which is an intraoral sensor that receives the x-rays that have passed through the tooth.
This sensor comprises: a scintillator on entry to convert the x-rays that have passed through the tooth into visible light; and a fiber optic plate to transmit the converted visible light to a charge-coupled device CCD-type detector, which converts the converted visible light into an analog electrical signal, while absorbing the residual x-rays that have not been converted into visible light.
The electrical signal is amplified and transmitted in analog form through a long cable, to a remote processing and display workstation where the signal is digitized and processed to produce an image that is then viewed on a display screen.
This type of equipment with a charge-coupled device detector creates a high signal-to-noise ratio (SNR), for example, of about 60 dB.
Also known, according to U.S. Pat. No. 5,912,942, is a type of x-ray detector wherein the active pixel sensor (APS) uses CMOS manufacturing technology.
In the above-mentioned patent, the radiology equipment described therein comprises: a source of x-rays passing through an object, a scintillator that converts the x-rays that have passed through the object into visible light, and a fiber optic plate that transmits the converted visible light to an active pixels array that converts it into an analog electrical signal.
It can be observed that the CMOS detector obtains a signal-to-noise ratio (SNR) of inferior quality to that of the CCD detector.
Several factors have thus been identified that limit the signal-to-noise ratio of the CMOS detector.
Among these factors, is the dark current that can be defined as being the electrical current collected at the detector output when the latter is not exposed to any x-ray.
The presence of the dark current leads to a deterioration of the signal-to-noise ratio.
It may be noted that, insofar as the intensity of the dark current has the special feature of considerably increasing with temperature and as the detector heats during its use, it is advisable to cool it and/or not operate it for too long a period so as not to further deteriorate the signal-to-noise ratio.
A second limiting factor is the detector's fill factor.
For a CCD detector, the fill factor is in theory 1, which means that the whole pixel surface is used to capture the x-rays and produce the corresponding electrical charge that will contribute to forming the image of the x-rayed tooth.
On the contrary, in a CMOS active pixel detector, the active element of the pixel occupies part of the pixel surface, without however contributing to the capture of the x-ray.
With part of the pixel not contributing to the fill, i.e. not contributing to the photon-electron conversion, the fill factor is less than 1, which hinders obtaining a good signal-to-noise ratio.
A third limiting factor results from the fact that today we do not know how to use CMOS technology to make large size monolithic active pixel arrays, typically about 20×30 mm, which are the commonly used dimensions for dental radiology sensors.
To obtain a large-size active pixel array in CMOS technology, it is necessary to assemble together several smaller-size sub-arrays by “stitching”.
The unevenness created by an array obtained in this way contributes to deteriorating the signal-to-noise ratio.
Further, it should be noted that the conditions specific to the field of dental radiology make the design of dental radiology apparatus with a high quality signal-to-noise ratio particularly difficult.
In particular, insofar as people are exposed to x-rays, the x-ray doses used should be as low as possible and these people should be exposed to the x-rays for the shortest possible time.
In other fields where a CMOS technology x-ray detector is used, the x-ray doses are not required to be as low as in dental radiology, which enables a higher intensity signal at the detector output, and thus a better signal-to-noise ratio.
Also, one of the special features of intraoral dental radiology sensors stems from the fact that the sensor that is placed in a patient's mouth has to be as small as possible to limit the discomfort caused to the patient, which implies reducing the number of components in the sensor.
It may be noted that, in a preferred embodiment of the x-ray detectors described in U.S. Pat. No. 5,912,942, the sensor comprises an integral analog-digital converter in order to digitize the analog output signal that is to be transmitted to the remote computer without delay.
Detector design like this goes against the miniaturization required for installation in a patient's mouth.
In addition, the introduction of an analog-digital converter alongside a CMOS technology active pixel array, which is an analog element, constitutes an additional source of noise which, added to the constraint of a minimal dose of x-rays, contributes to deteriorating the detector's signal-to-noise ratio.
Other unmentioned sources of noise may be noted that are also capable of affecting the detector's signal-to-noise ratio.
Generally, there is a need for new dental radiology apparatuses and signal processing methods used therein, which can improve the signal-to-noise ratio provided by the apparatus's detector.
It is also apparent that the use of existing dental radiology apparatuses causes hygiene problems that it would be desirable to solve, at least to some extent.
Thus, when the dentist wearing surgical gloves places an intraoral sensor behind a tooth in a patient's mouth, said sensor comprising one of the detector types mentioned above, he/she has to switch on the sensor and then start the x-ray generator.
To do this, he/she has to go to the computer located a few meters away, which is already not very practical, and then click on a computer mouse to start the sensor and the x-ray generator by means of a programmed interface.
However, at this time, the dentist is wearing gloves that are already contaminated by the patient's saliva, which risks causing cross-contamination when the dentist later handles the computer mouse using gloves impregnated with another patient's saliva.
Faced with such a situation, the dentist has then either to remove the gloves before handling the mouse, or disinfect them after use, which in both cases represents additional constraints that quickly become onerous when they are repeated several dozen times a day.
On the other hand, there is also a need to have dental radiology apparatus as small as possible, especially for the intraoral sensor and related electronics.
Thus the invention aims to remedy at least one of the above-mentioned problems.